Etching



United States Patent ETCHING Marvin H. Fishaber, Saginaw, Mich assignor to The Dow Chemical Company, Midland, Mich, a corporation of Delaware No Drawing. Filed July 31, 1956, Ser. No. 601,116

Claims. (CI. 41-42) This invention relates to a method of etching metal. It more particularly concerns an improved method of producing a relief in the surface of an object easily soluble in acid, as in-making a printing plate by etching with an acid solution. 7

A new etching process was introduced into'the art, and one which is particularly adapted to photoengraving, upon the issuance of Patents Nos. 2,640,763 to 2,640,767, inclusive, on June 2, 1953, to John A. Easley and Harry E, Swayze. In this new etching procedure, the bath composition is so controlled and the bath itself is impinged upon the photoengraving plate. in such a manner that lateral etching and/or undercutting of the image areas is reduced or, in some cases, eliminated. A major advantage of this procedure lay in the fact that the time consuming powdering techniques were substantially eliminated. a I

In the commercial use of the above-noted etching patents, it was found that the bath composition must be varied to obtain optimum results depending upon the type of image that is to be etched. In linear areas, where the image to be etched comprises printed matter or images containing only solid lines, it has been generally found that the image must be etched to a depth of at least 0.020 inch. In such cases undercutting is a problem and the amount of water-immiscible organic liquid employed must be maintained at a relatively high level. When treating photoengraving plates containing halftone areas, which comprise images made up of a plurality of very small dots which produce the effect of shading due to variation in the size of the dots on different por-v tions on the plate, it is necessary to etch only from 0.0025

inch up to 0.007 inch, depending upon the screen ruling employed, the amount of water-immiscible organic liquid used in these baths may be reduced to a minimum. Line areas and half-tone areas, by themselves, present no great problem in the use of the new process so described, but it has been found to be very difficult to etch plates known in the trade as combination plates, which contain both half-tone areas and line areas. This problem is due to the fact that the depth to which line areas are generally etched is many times greater (by a factor of 3 to 10) than the depth to which half-tone areas are normally etched. While these combination plates represent a highly specific problem in the photoengraving art, they are still representative of an important phase of this art because of the large numberof such plates used in newspapers. 4

It is an object of this invention to provide an improved method and bath for etching acid-soluble metal objects, particularly developed photoengraving plates which bear a resist image. Another object of this invention is to provide an improved method and bath for etching 'developed photoengraving plates containing both half-tone and line areas.

It has been discovered that the etching action of etching baths, such as defined in the above-cited patents to Swayze and Easley, may be greatly enhanced by adding thereto a sulfur-containing compound soluble in aqueous dilute nitric acid solution selected from the group consisting of inorganic sulfates, organic sulfates and sulfonates in which the organic portion is selected from the group consisting of alkyl radicals containing not more than 8 carbon atoms, naphthenic radicals, phenyl radicals,

and alkyl phenyl radicals wherein the alkyl portion comprises one or more alkyl groups and contains a maximum The inorganic sulfates include sul-- of 6 carbon atoms. furic acid, sodium sulfate, sodium acid sulfate, magnesium sulfate, aluminum sulfate and ammonium sulfate solution in the bath in amounts such that the sulfur content of the solute ranges from 0.02 to 0.3 grams per liter of bath solution. This range defines critical limits for sulfuric acid and sodium sulfate, but it will be apparent to those skilled in the art that certain variations in this range will occur depending upon the particular sulfurcontaining compound employed. Also, the sulfur content of the solute may be in part made up from sulfur derived from adventitious elements, so that the amount of sulfur-containing compound added may vary, for example, with the sulfate content of the water employed. In order to derive the enhanced etching action required to produce satisfactory combination plates, sufiicient sulfur-containing compound must be added so that the sulfur content in the solute ranges from 0.03 to 0.18 grams per liter of bath solution, and the preferred range is from 0.04 to 0.095 grams per liter. The present invention is all the more remarkable when it is considered just how small a percentage of the sulfur-containing coma pound is required to produce the desired results. The addition of an equivalent amount of nitric acid produces no perceptible distinction in the characteristics of the bath.

Although the ability of. the sulfur-containing com- 7 In applying baths comprising thecpresent invention to the etching of combination plates, .it has been found" necessary to maintain ,anextremely delicate balance betweena number of factors inorder to produce commercially acceptable plates. For this purpose, the commercially operable baths are thosewherein the ingredients .are maintained within quite narrow limits.

Experimentation has shown that when this bath is to be employed in the etching of combination plates the nitric acid content should bemaintained between 3 and,

15 percent by volume measured as 42 Baum acid; As

used in this specification and in the attached claims the term "percent by volume" refers to the total volume of i the etching bath.

Nitrie'acid"as used herein refers in each instance to 42' Baum acidand concentrated sulfuric acid is defined "as reagent grade acid having a specific gravity of 1.84. When the concentration of the nitric acid falls below 3" p ercentthe time required for etching th plate is unduly: increased; while above :15 percen t undercutting of the image areanaay occur. It is preferred] that the nitric acidcontent bemaintained'in therange of 5.0: to l1.0p ercent by volume. V A

above set forth',,, the wettingagents that we prefertdeinploy-are thoseidescrihed and claimed in Patent Number 2,'640,765.- When the bath is to be used to etch combination plates; however-,it is necessary-to these wetting agents 'to an ester ofsulfosuccinic acid; and an; aliphatic alcohol having fromfl to'9 earbcimatonns. Ofthese'este'i's, we prefer to employ clioctyl sodium sulfosuc- 1 cinate; The amount" of; wetting agent employedranges from 'ZGt'o )1grams per liter.- Below 2 grams per liter t ofwetting-agent;undercutting niay-occur.- Whenthewet-i ting agent is; employed in amounts *greater than g'ramsa'per liter of bath solution, a roughunevenietchmg ofthef-nietalsurface'may occur.

' For general purposes; any waterqmmiscibleorganic liquid which is substantially unreactive with the aqueolisnihietacid solu'tion'maybeemployed; V For combination plates, however, it is preferred to enipIoy'liquid allcylbe'nzenes or mixtures thereof 'wh'erein the alkyl portion is composed of oneor. 'morealkyl 'groups having a maxi a mum of 4 carbon atomsper group}, and the' tot'al num= 7 her 1 of carbon atoms inthe alkylportion does not exceed .0,: These-liquid alkyllienzei'lesv include: such compounds higher or lower temperatures. The length of time required to' etch a combination-plate using the preferred bath compositions above set forth will vary from 7 /2 minutes in a fresh bath to minutes in a used bath. A deep etch bath, such as that disclosed in US. Patent No. 2,640,765, may be employed to increase the depth of etch in the line areasandnpe'n areas.

The following examplessare illustrative of the present invention:

v Example-I L H p A plate is inches-by'24 'inches 'by"0.0 64 inch formed of a magnesium-base alloy having a nominal composition of apar i a y .V3jipm t um num, Loperc nnzin the image of the printed matter and the half-fonearea as nonoethylben'zene;v the diethylbenzenes; the dimethyl benzenes, ;the1 tetramethylbenzenes; the tetracthylbenzenes and the-monobutylbenzenes; Thenamount' of alkyla benzene employed may vary from; 2.0 percent. to 4.5

perce ntlby' volume. An: uncven,i etchingiwith excessive filmingis obtained when; les s than; 2.0 percent. by. volume ofi alkylben'zene is ,employed and, a, eras per;

cent is preferred.- When moreathan 3.31percent1alkyl-. V

' benzene. is used an inadequatedepth ofetch: is generally" obtained inlthe half-tone areas/although up to 4.5 per-j cent by volume of alkylhenzene can Jae-used andstill;

ditions. V t t In' etching photoengraving' plates with baths-of. the} produce acceptable -combinationplates; under some 0011? present invention,. commercial practise has shown that it is} generally necessary is condition,v the baths by: dissQIY BgI magnesium therein in order to obtain a -plate;

that, will be satisfactory for; printingpurpgsesqz Although:

continued use of the bath will itself effect the required; conditioning, initial conditioning is fies sential if the :first fdill Pl tes etchedin thelbath are to becommercially.

useable; It has beenidiscoveredthat the addition of. a

soluble nitrate salt, such'as magnesium nitrate orv sodium ee p t se' w aa e o, about; 310.5 grams .per, liter; The bestmanner of defining bath' conditioning The half-tone area wasetched to adepth of .006 Finch;

rQ m nes um-di so vedjn t gbathiorlcondi factor is by reference to 'the nolar ratio betweenjthe hydrogen-content of the nitric acid and the' total nitrate; radicalpresent in the bath. The nitrate-addition, or dis-"Ii solution (pf-magnesium, should be so regulated that this": molar ratio of hydrogen'tomitratewill rangefrom 0.3 to 0.9'and preferably between 0.4 and 0.8. V V The temperature of: the bath is preferably maintained ,between 'and iIt will be apparent to those skilled in the art; that as {the temperature variesgrthe com positionof -th --bathmust be altered to-eompensate for six;.1ninute's'", .the oven temperature being t.

the samemagnesium-base o theiremainder ot the plate ,beingbarel r'Ihelresist image name la e. was .setby heatingftheiplatetinnantoven for 450,? Tue plate wast-then descummed by a 2 s immersion ingan' aqueous" solution iconsisti'ng of lz jzi/(Q Water, sufiicient to-make' .134liters.

v A niaterial comprising 75ipe'rcent' hy wei lit- 01 .dl'octyli sodlum-sulfosncc inatey, 20 percent ,watertnnd: percent ethyl alcohol. V I V v The bath. was conditioned by etching f lsi inchrby 24 inch sheet 'of magnesium. photoengraving plate for about. 18 minutes at. 600 r.p.ni. paddle;speed, and ,thus ,dissolv-u ing 600 grams; ofmagnesium inthefhath. r v v The above bathlformulation was'placedjn an etching. machine corresponding .to that disclosedin vPatent Nor" 2,665,048jand maintained at atemper'ature'72""'F. Theplate' was held with the ima e 'h'earing side' down .at a, height 'ofjabout v3 "inches above the uppermostjpqifit' or travel of theftips of 'the ,.paddles 7 These paddles were;

positioned in the bath andtsplalshedfthe 'ba'th'iupwardlyf.

for-one-ha'lf minute, with" final reduction in paddle speed to "380"r.p;rn;: for aperiod of 7 minutes; The etched plate; was rinsed and -dri-ed': and"the large fhare portionsw thereof were found to "be etched *to "a depth inane highlight-areas and showed no 'undercutting or ldssf of dots. In type of about 8 point sizethe're was anetch depthofgO-lS iinch 'in' theqareas 'hetweenthe-l r '5 I niiwaiilai-gf "lUsingr-aplate l inclie' j, I f v V V V e p H 7 y and'coatedwith the'samef r'esistas defined in Examplel, all of theproeeduresjqf. Examplel were, followed except .for. a difierenttetclii'n'g" bath and'fsome variation-in the .paddle" speed 'employ ed."

' bath *used' hadjthe" "following composition: J V

Nitric acid 42 Bay .,liters' -i .16 Dioc'tylsuccinate ester' gram s 950. Diethylbeuzene v liters; .4;;15 Sodium sulfate (anhydrous) grams 50 Water,..sufiicient to make 134 liters. 1

t defined emmas-1 on the basis of sulfur content, was 0.0268 gram/liter in Example 1 using H SQ; and 0.084 gram in Example 2 using sodium sulfate, (anhydrous) The amount of sulfur-containing compound, nitric acid, wetting agent and water-immiscible organic liquid may be varied within the ranges above set forth. Plates of various compositions may be employed including zinc plates, plates composed of zinc alloys containing over 90 percent zinc, magnesium plates, and plates composed of magnesium alloys containing over 80 percent magnesium.

It will be apparent to those skilled in the art that the invention above described may be used to advantage in a variety of ways. The bath comprising the present invention may be employed to etch combination plates, plates containing only line areas, and plates containing only half-tone areas, although it is unusually effective in etching combination plates. By the combination of an enhanced etching action arising fromthe addition of the sulfur-containing compound to the bath and by careful control of the velocity with which the bath impinges upon the plate to be etched, combination plates have been etched without requiring any powdering. This is achieved by operating the paddles in the bath at a relatively high velocity in the initial stages of etching, this velocity of impingement of the bath on the plate being sufficient to rapidly etch the half-tone area. When these half-tone areas have been etched to a satisfactory depth, which is normally from 0.0025 inch to 0.007 inch, the speed with which the paddles are rotated in the bath may be reduced so that the velocity of impingement of the bath on theplate is such that the etching of the line areas is continued and substantially no etching occurs in the half-tone area. This selective etching of the action bath is critical in producing commercially acceptable plates.

I claim:

1. An etching bath comprising (I) a water-immiscible organic liquid substantially unreactive with the following aqueous solution, and an aqueous solution containing (H) nitric acid, (III) a wetting agent selected from the group consisting of: (1) sulfosuccinic acid esters prepared from aliphatic alcohols having from 4 to 12 carbon atoms; (2) petroleum sulfonates containing from 18 to 30 carmon atoms; (3) saturated aliphatic acids containing from 5 to 26 carbon atoms; (4) esters of polyhydric alcohols containing from 2 to 6 carbon atoms and acids containing from 12 to 18 carbon atoms; and (IV) a nitric acidsoluble sulfur-containing compound selected from the group consisting of (A) inorganic sulfates and (B) organic sulfates and sulfonates wherein the organic portion is selected from the group consisting of (a) alkyl radicals containing not more than 8 carbon atoms, (b) naphthenic radicals, (c) phenyl radicals and (d) alkylphenyl radicals wherein the alkyl portion is composed of at least one alkyl group said alkyl group containing a maximum of 6 carbon atoms.

2. An etching bath comprising (I) 2.0 to 4.5 percent by volume of a water-immiscible organic liquid substantially unreactive with the following aqueous solution, and an aqueous solution containing (11) 3 to 15 percent by volume of nitric acid, (B1) 2.0 to 10.0 grams per liter of a wetting agent selected from the group consisting of: (1) sulfosuccinic acid esters prepared from aliphatic alcohols having from 4 to 12 carbon atoms; (2) petroleum sulfonates containing from 18 to 30 carbon atoms; (3) saturated aliphatic acids containing from 5 to 26 carbon atoms; (4) esters of polyhydric alcohols containing from 2 to 6 carbon atoms and acids containing from 12 to 18 carbon atoms; and (IV) a nitric acid-soluble sulfurcontaining compound in an amount such that 0.02 to 0.3

6 grams of sulfur per liter of bath results and wherein the sulfur-containing compound is selected from the group consisting of (A) inorganic sulfates and (B) organic sulfates and sulfonates wherein the organic portion is selected from the group consisting of (a) alkyl radicals containing not more than 8 carbon atoms, (b) napthenic radicals, (c) phenyl radicals and (d) alkylphenyl radicals wherein the alkyl portion is composed of at least one alkyl group said alkyl group containing a maximum of 6 carbon atoms.

3. An etching bath comprising (1) 2.5 to 3.3 percent by volume of a water-immiscible organic liquid substantially unreactive with the following aqueous solution, and an aqueous solution containing (II) 5.0 to 11.0 percent by volume of nitric acid, (III) 2.0 to 10.0 grams per liter-of a wetting agent selected from the group consisting 'of:-

(1) sulfosuccinic acid esters prepared from aliphatic aloof. hols having from 4 to 12 carbon atoms; (2) petroleum sulfonates containing from 18 to -30 carbon atoms;

(3) saturated aliphatic acids containing from 5 to 26 car bon atoms; (4) esters of polyhydric alcohols containing from 2 to 6 carbon atoms and acids containing from 12 to 18 carbon atoms; and (IV) a nitric acid-soluble sulfurcontaining compound in an amount such that 0.03 to 0.18 grams of sulfur per liter of bath results and wherein the sulfur-containing compound is selected from the group consisting of (A). inorganic sulfates and (B) organic sulfates and sulfonates wherein the organic portion is selected from the group consisting f (0) alkyl radicals containing not more than 8 carbon atoms, (b) naphthenic radicals, (c) phenyl radicals and (d) alkylphenyl radicals wherein the alkyl portion is composed of at least one alkylgroup .said alkyl'group containing a maximum of 6 carbon atoms. l

4. An etchingbath comprising (1) a water-immiscible organic liquid substantially unreactive with the following aqueous solution, and an aqueous solution containing (11) nitric acid, (III) sulfosuccinic acid esters prepared from aliphatic alcohols having from 4 to 12' carbon atoms; and (IV) a nitric acid-soluble inorganic sulfate.

5. An etching bath comprising (I) 2.5 to'3.3 percent by volume of diethylbenzene and an aqueous solution containing (II) 5.0 to 11.0 percent by volume of nitric acid, (III) 2.0 to 10.0 grams per liter of sulfosuccinic acid esters prepared from aliphatic alcohols having from 7 to 9 carbon atoms; and (IV) sodium sulfate in an amount such that 0.03 to 0.18 grams of sulfur per liter results.

6. A process for etching a developed photoengraving plate containing both line and half-tone images thereon and consisting essentially of a metal of the group consisting of magnesium, magnesium-base alloys containing over percent magnesium, zinc and zinc-base alloys containing over percent zinc, which comprises impinging upon the surface thereof at a velocity suflicient to rapidly etch the half-tone areas, an etching bath comprising (1) a water-immiscible organic liquid substantially unreactive with the following aqueous solution, and an aqueous solution containing (II) nitric acid, (III) a wetting agent selected from the group consisting of: (1) sulfosuccinic acid esters prepared from aliphatic alcohols having from 4 to 12 carbon atoms; (2) petroleum sulfonates containing from 18 to 30 carbon atoms; (3') saturated aliphatic acids containing from 5 to 26 carbon atoms; (4) esters of polyhydric alcohols containing from 2 to 6 carbon atoms and acids containing from 12 to 18 carbon atoms; and (IV) a nitric acid-soluble sulfur-containing compound selected from the group consisting of (A) inorganic sulfates and (B) organic sulfates and sulfonates wherein the organic portion is selected from the group consisting of (a) alkyl radicals containing not more than 8 carbon atoms, (b) naphthenic radicals, (c) phenyl radicals and (d) alkylphenyl radicals wherein the alkyl portion is composed of at least one alkyl group said alkyl group containing a maximum of 6 carbon atoms.

a 7 7. process for etching ardeveloped' photoeiigraving. platercontainingi both line and halt-tone images thereon aint consisting .ess'entiallyiof a'metal of the groupcone sisting of-ima'gnesium,r rna'gneSunni-base alloys containing" over SO iJercenLmagneSium, zinc'and zincrbase alloys containing over 90 percent zinc, which comprises impinge ilig liponllhfl surface thereof at a velocity sufficient to rapidly: etch the half-atone areas,lan etchingbathj comprising-1(1) 2.0' to 415 I percent by- Vvolume o'f a waterimmiscible organic liquid substantially unreactive with the following. aqueous; solution; and an aqueous solution containing (11). 3" to. 15' percent byr'volurne 10f nitric acid, 4

(HD51 i to::l0.0 grams per. liter lofia' wetting agent selectedaironrithefgroup consistinglzofz; (1). sulfosuccinic" I acidfesters prepared from aliphatic aleoholsihaving from 4am l2ficarbon atoms; (2) petroleum sulfonates containingiroinzliiito 30*.ca'rbonatoms; 1( 3 saturated. aliphatic. v, acids containingrfrorn 5 :to 26 1:arbon atoms; .(4)"esters of pol-yhydric alcohols containing from; its "6 carbon atoms acids? containing fiom'l 12: to =18: carbonatoms; and

a a 8 atoms'Eandf acids containifig from 12516 l8 carbon and (IV) Fa? nitric acids'c'iluble: sulfur-contalmi "ng' com poundan -jamountfsuclr that. 0.03 to 0J18"gramsof sulfur'per liter of bathiesultsf'and whereinf ther sulfur containing compound istselected' fionrthegroup consistin'g 0131A) inorganic zsulfatesl and (3). organic sulfates (1V1)? a; nitric.acid solubleisnlfuricontainingi compound in:

an; amount'such that 0.02 to 0.3 gram's:of sulfurfiper liter oflbath-iresults and i-wherein the lsulfuracontaining; com 7 pound. is'iselected fromthe group consisting of (A) inorganicisHlfatesandKBT-I organic sulfates and sulfonates V whereinltherorganicportion is selected from: the groupi consisting of (1) alkyl radicals containing not more than- 8-Icarbon:atoms,i(h') naphthenic'radicals, (c)lphenyl radicalsz'and (Zllj' alkylplienyl' radicals wherein Lthetalkyl 'pprg tiorris comp'osedl of: at least: one alkyl group; said ialkyl: groupcofitainingfa maximum of:6 ca'rbon-latoms. ,1 T 1 7 {8i A'zp'rocessi or; etching a deuelopedv photoengraving" 7 platezconta'ining botlriline andl'half-tone iimages thereon f and consisting essentially of a metal of thegrouptconsist 'ing la-of'i'rnagn'esium, magnesium-base alloys containing 'over8 percent.magnesium;.zinc andrzinc=base all'oys con tainingrover 1 90' percent 'zinc; which comprises impinging V upomthsutfacelthereof'atai'uelocity sufiicient to'rapidly etch: the": halt-tone areas, LBIL etching "bath comprising: (I) 2.5 toi3r3 -percent'by'yolume ofa water-immiscible:

organielii uid substantiallyiunreactive .with thexfollowing aqueous solution, and an aqueous solution containing (II)? 510 tor 11.0 percent by volume of nitric'acid, (:III)v2.0ito. 10.0 grams 'perrliter of a wetting agent selectedifromfithe grdupbeesisting ofi (1)7 sulfosuccinic acid esters-prepared from aliphaticalcohols having from 4"to 12"carbonatoms; (2) petroleum sulfonates contain ing from 18 to carbon atoms; (3) saturated aliphatic aeidscontaining from 5 to 26 carbon atoms; (4) esters ofpolyhgidrieralcoholsfcontaining from 216 6 carbon? andsulfonate's whereifilthe organic'portion is selected fi'omztheigroup consistingloi a)- alkyl radicalsicontaining' notmoregthan Scarbon atoms, (be) naphthenici'radicals; (c phenylfradicalsjand (d) alkylphenylradicalswhere in the alkyl' portion is:,compiosed ofat least one-:alkyl group said alkyl r group, containing-l a maximum of" 6 carhon atoms. I 1 j i i i9; :A" process "for etchinga developed photoengraving plate containing both. line.;ancl'zhalf-ton images thereon I and consistinggessentially; of, a" metal of the groupconsistingi ofiomagnesium, magnesiumebaseralloysicontaimng over 80i=percent niagnesium; zinc and zinc-base -ralloyscontainingtover': 90ipercentlzinc; which-:comprisesimpingin'g' 7 upon theisurfacethereof atia velocity: sufficient to rapidly etch. the halfrtone areas, an etchin'g lbath comprisingiI) :a'

' water-immiscible Zo'rganicliquid substantially; unreactive;

with theifollowinggaqueous solution;- and an laqueous so.--

' lution containing fll) nitric:acid,s(III) .-sulfosuccinic acid esters prepared from-aliphatic aloholshavingiromto 12- carbon: atomsi and-(lv) a nitric acid-soluble inorganic V sulfate -r 1 210. A ramelror' ;etchin gra deuellop ed photoengravi-ing.

v 6 plate containing bQth-line'and ehalfztoneiimages thereon and consistingzessentially ofarnetalofiith'e-gr'oup consistingrof magnesiumrmagne'siumebase'alloys containing;over "80; percentmagnesiumazinc; and zinc-base alloys contain-l V -in'grolvera90 percent 2inc;which, comprises'iimpinging upona the surface-thereof: at a:velocity. suflicienfl torapidly; etch;

the hal-f-tone areas; .an etchingtbath comprising'fl). 2.5 to; 3.3 :percent'bywolume of diethylbenzene and an aqueous solution containing; (II) 54010;1 1 .05 percentbywolume .of- 

5. AN ETCHING BATH COMPRISING (I) 2.5 TO 3.3 PERCENT BY VOLUME OF DIETHYLBENZENE AND AN AQUEOUS SOLUTION CONTAINING (II) 5.0 TO 11.0 PERCENT BY VOLUME OF NITRIC ACID, (III) 2.0 TO 10.0 GRAMS PER LITER OF SULFOSUCCINIC ACID ESTERS PREPARED FROM ALIPHATIC ALCOHOLS HAVING FROM 7 TO 9 CARBON ATOMS, AND (IV) SODIUM SULFATE IN AN AMOUNT SUCH THAT 0.03 TO 0.18 GRAMS OF SULFUR PER LITER RESULTS. 